Good to be fluent in both. I only have pistols on which I can reach the slide release with my thumb without rotating the grip in my hand; therefore, I train using the slide release, which is faster. Having said that, I also spend time in training to clear Type 1, 2 & 3 malfunctions with one hand and both hands.
Food for thought #1 – What gun? If you can reach the slide release on your XD as easily as I can reach my releases on my XDm’s and other pistols, then use the slide release. With training, your thumb will be in position to drop the slide as soon as the magazine is seated. In fact, if you train enough, you can learn the proper amount of pressure to place on the release which will cause the slide to drop when you slam the mag in place. However, if you are shooting, let’s say a 1911, where you cannot easily reach the slide release, it might be better to train in racking the slide using the overhand method. I could see there might be problems in the “fine motor” skill in getting your support hand in the act of dropping the slide (something which should be a problem if you train enough).
Food for thought #2 – How much training? If you are shooting a few hundred rounds per year, coupled with no dry fire practice, then it could be best to stick with “gross motor” slide manipulation with the support hand; however, if you are averaging 50 to 100+ rounds per week, along with dry fire work, then by all means train to use the faster slide release method, with alternate methods added (such